1. Field of the Invention
The present invention relates to the preparation of a fuel from a waste sludge that contains water, solids and oil by means of filtration and drying. It relates in particular to the use of the dried filter cake obtained by such method as fuel in a cement kiln, or other combustors.
2. Description of the Related Art
Waste streams are a growing concern to the environment. Authorities impose increasingly stringent requirements as to the disposal of waste streams. In refineries, waste sludges are obtained as a waste stream during the work-up of crude oil fractions. Such sludges may include water, salt, catalyst wastes, dirt-like sand, and heavy oil. Sludges like these are not allowed to be disposed of without pre-treatment.
A suitable pre-treatment comprises a filtration to separate the solids from the liquids so that the solids can be disposed of and the filtrate can be worked up. One of the problems that arises in connection with disposal of the resulting filter cake relates to the leachability of metal ions therefrom. One way to solve the leachability problems resides in the fixation of the metal ions in cement. The waste may be mixed with cement and the admixture is allowed to set and a solid hard material is obtained which passes the leachability requirements set by the authorities. Another way to solve the problem of the metal ions is to incorporate the metal ions into the cement. Hence, it has been proposed to use solid material resulting from the filtration of waste as fuel in cement kilns so that the hydrocarbons can be burned and the metal ions contained in such material would be retained in the cement produced in the kiln. Cement producers, however, set certain requirements as to the quality of the material to be used as fuel. One such requirement is that the heating value of the material is at least 5,000 Btu/lb (about 11.6 kj/g). A higher heating value would be even more beneficial to the operation of the cement kiln since it will take 3-6 MM Btu energy to produce one ton of cement Another requirement is that the moisture content in the material should not exceed 10% wt, based on the total material. A third requirement is that the material is non-sticking to enable pneumatic transportation.
In a known treatment of refinery waste sludges the sludge is usually filtered at considerably high pressure, such as up to 200 psia, (about 13.7 bara) or subjected to severe centrifuging, thereby squeezing all fluids from the solids thus separated. The solids obtained are then dried at temperatures up to about 450.degree. F. (about 232.degree. C.). The remainder is a non-sticking solid material containing only a small portion of the oil present in the waste sludge. The material generally has a heating value of between 500-4,000 Btu/lb, which will not be sufficient to be used as solid fuel for a cement kiln or other combustors.
Bartilucci (U.S. Pat. No. 5,009,767) discloses a method in which oily sludges and other refinery waste streams are recycled, by segregating refinery or other sludges, to produce a high oil content waste, which is injected into a delayed coking unit, during the coking phase of the cycle and a high water content waste which is injected during the quenching phase of the delayed coking cycle. The high oil content waste is preferably subjected to a filtering operation prior to injection into the coker drum in order to remove water as well as components which increase the ash content of the final coke. However, Bartilucci fails to produce a high Btu, nonsticking solid fuel.
Greig (U.S Pat. No. 4,260,489) discloses a process for the treatment of oily sludge by filtering it through a precoated surface filter, treating the filtration residue with a hydrocarbon solvent and/or steam stripping the extracted residue. The residual oil content of the treated sludge is very low and it can be used for land fill operations.
An advantage of the present invention's process over the prior art's process is that the present invention's process reduces the moisture in a waste sludge to about 10% wt or less, while maintaining the maximum amount of oil in the waste sludge. This waste sludge has a high heating value which will be sufficient to permit use as a solid fuel for a cement kiln or other combustors.